An amorphous silicon thin film transistor (a-Si TFT) has been widely used as a switching device of an image display device such as a liquid crystal display (LCD) device, etc. However, recently, research for using a polycrystalline silicon (hereinafter, polysilicon or poly-Si) TFT having an operation speed faster than that of the amorphous silicon TFT as a switching device has been actively pursued as the desire for high picture quality LCDs has increased.
To fabricate a polysilicon TFT, a polysilicon layer may be formed by heat treating an amorphous silicon layer. Either a high temperature furnace or an excimer laser may be used to provide the heat treatment.
However, since an LCD uses a glass substrate, which is deformed at a temperature of greater than 600° C., it is difficult to crystallize an amorphous silicon layer on the glass substrate in a high temperature furnace. Accordingly, crystallization using an excimer laser is generally used. The excimer laser irradiates the amorphous silicon layer using a high energy laser beam. The laser provides instantaneous heating over several tens of nanoseconds, which is short enough to allow the glass substrate to remain substantially unaffected.
Another benefit of using an excimer laser to form the polysilicon layer is that the polysilicon layer has a higher electron mobility than that of a polysilicon layer formed by a furnace heat treatment. Generally, amorphous silicon has an electron mobility of 0.1˜0.2 cm2/Vsec. Whereas polysilicon formed by a furnace heat treatment has an electron mobility of 10˜20 cm2/Vsec, polysilicon formed by an excimer laser has an electron mobility exceeding 100 cm2/Vsec.
The polysilicon TFT fabricated by an excimer laser crystallization has an excellent electron mobility in an ON state, however a large amount of leakage current is generated in an OFF state. Therefore, it is desirable to reduce the leakage current in the OFF state of the polysilicon TFT formed by an excimer laser crystallization.
The leakage current of the polysilicon TFT is generated due to the following reason. In the OFF state, even if a voltage corresponding to approximately 5˜10V is applied between a source and a drain electrode, a high electric field is formed between the source and the drain region under a state such that a current does not flow between the source and the drain electrode. In the OFF state, an electron-hole pair is generated at a grain boundary where bonding between silicon particles is relatively weak. The generated electron-hole pair separates at the boundary, thereby producing a leakage current.
Moreover, an inner grain boundary of the polysilicon layer causes the electron mobility of the device to be lowered in both the ON and OFF states. This is because at the grain boundary, the bonding between silicon particles is cut or silicon particles are incompletely bonded to each other, which prevents the flow of electrons or holes. Thus, even if the polysilicon TFT has a higher electron mobility than that of an amorphous silicon TFT, the electron mobility of the polysilicon TFT is lower than that of a single-crystalline silicon TFT because of the grain boundary.
In order to solve these problems, the density of the grain boundary is lowered by increasing the grain size. In order to increase the grain size, an intensity of the laser energy is increased or the substrate is heated.
Referring to FIG. 1, the intensity of the laser energy is proportional to the grain size in a first region and a second region. However, a small grain corresponding to 100 nm is grown in a third region due to the following reasons.
When the laser beam irradiates the surface of the amorphous silicon layer, a portion of the surface of the amorphous silicon layer directly exposed to the laser beam is irradiated by a high intensity laser beam, and a relatively weak laser beam irradiates a lower portion of the amorphous silicon layer. Accordingly, the surface of the amorphous silicon layer directly exposed to the laser beam is completely melted while the lower portion of the amorphous silicon layer is melted incompletely. Since a grain grows centering around a seed, for example impurities or incompletely melted amorphous particles, the lower portion of the amorphous silicon in the incomplete molten state serves as a seed to grow a grain of a large size centering around the seed.
When an intensity of the laser beam is at more than a critical level, the amorphous silicon is completely melted and a seed for growing a grain does not exist. Then, in a cooling process, a seed is randomly generated in the melted amorphous silicon and the amorphous silicon is crystallized centering around the seed.
At this time, the generated grain has a very small size, as shown in the third region of FIG. 1.
In the process of cooling, the amorphous silicon is cooled through both lateral surfaces on which a laser beam has not been irradiated. This is because the solid amorphous silicon layer of the lateral surfaces has a greater heat conductivity than the amorphous silicon layer of the lower portion.
The completely melted amorphous silicon is crystallized from the unmelted amorphous silicon of the lateral surface. At this time, the unmelted amorphous silicon serves as a seed for crystallization and the crystallization is performed in a lateral direction with a certain pattern.
At a region that is not in contact with the solid amorphous silicon layer that serves as a seed in the silicon layer melted by a laser beam, a minute crystal is randomly grown in a cooling process. The minute crystal serves as a seed for a grain growth to perform crystallization.
FIG. 2 shows a state of an amorphous silicon crystallized by the above crystallization. Referring to FIG. 2, the crystallization method will be explained in more detail.
First, a part of an amorphous silicon is shielded by a mask, and then a laser beam irradiates the amorphous silicon. The amorphous silicon region shielded by the mask is not melted, but the amorphous silicon region on which a laser beam has been irradiated is completely melted and then is cooled.
The amorphous silicon melted in a cooling process is laterally crystallized by making the solid amorphous silicon of a lateral surface as a seed, and the melted amorphous silicon of a region not in contact with the solid amorphous silicon grows a small grain corresponding to several hundreds of nm centering around an arbitrary seed. The laser beam has an intensity that is strong enough to completely melt the irradiated amorphous silicon.
The crystallization is performed sequentially in a lateral direction. This is called sequential lateral solidification (SLS). As shown, the laterally grown grain has a size corresponding to 1˜1.2 μm.
Whereas a grain formed by a general laser crystallization has a size corresponding to several hundreds of nanometers, a grain formed by the SLS method has a size corresponding to several micrometer (μm). Therefore, if polysilicon obtained by the SLS method is used in a device, a device having a large mobility can be realized.
When a maximum size of a grain that can be grown by the SLS method is desired, the SLS method is performed from both directions, thereby obtaining a large crystalline structure having one large grain boundary in the middle part thereof.
FIG. 3 is a view showing an aspect of a grain grown by the method.
If the SLS method is performed by using a mask including an opening having approximately 2 μm, as shown in FIG. 3, the grain has one grain boundary and a laterally grown crystalline can be obtained. If the lateral grown polycrystalline is used as a channel of a thin film transistor, the thin film transistor has a high mobility.
A method for crystallizing an amorphous silicon by using the SLS method will be explained in more detail with reference to FIG. 4.
As shown in FIG. 4A, the SLS method is performed by using a mask 401 including an opening 402 having a width W and a length L as a laser beam shielding mask. Also, the SLS method is performed by scanning a substrate 400 in an X direction and by stepping the substrate 400 in a Y direction. The SLS method is performed by scanning a substrate with a bar type laser beam and moving the entire substrate in a zigzag form.
The unit moving distance of the substrate in the X direction is called an X direction scanning distance, and the unit moving distance of the substrate in a Y direction is called a Y direction stepping distance. The scanning distance is smaller than a width of the mask to partially overlap between adjacent crystallized silicon regions Also, the stepping distance in the Y direction corresponds to a length of a mask.
Hereinafter, the related crystallization will be explained with reference to FIGS. 4A to 4C.
As shown in FIG. 4A, the mask 401 including the opening 402 having the width W and the length L is aligned on the substrate 400. The mask may be formed inside or outside a projection lens of a laser generator, and the mask is aligned on the substrate by aligning the laser generator having the mask on the substrate.
After aligning the mask 401 on the substrate 400, a first laser shot irradiates the substrate 400 through the opening 402 of the mask. As the result, as shown in FIG. 4A, a laterally crystallized silicon region having one large grain boundary in the middle part thereof is obtained.
Then, as shown in FIG. 4B, the substrate 400 is moved in a −X scanning direction. As mentioned above, the moving distance is smaller than the width W of the mask. The reason is to partially overlap between a region crystallized by the first laser shot and a region crystallized by a second laser shot after moving the substrate in the X scanning direction. A larger crystalline silicon region may be obtained by overlapping the crystallized regions.
As the result, as shown in FIG. 4B, a crystalline region having a length of W/2+A can be obtained. The A denotes a length of the overlapped region after moving the substrate with scanning.
After moving the substrate with scanning in the X direction, as shown in FIG. 4C, the substrate 400 is moved in a Y stepping direction by a stepping distance and then the SLS method is continuously performed. The stepping distance is preferably smaller than the length of the opening 402 thereby to partially overlap the crystallized regions. The reason is for removing crystalline strain due to a grain boundary generated after the crystallization.
The substrate 400 is moved by the stepping distance, and then is continuously moved in the X direction, thereby performing the crystallization.
The crystallization is performed until the substrate is entirely crystallized.
However, in the related crystallization method, the crystallization is performed by using a mask having a fixed opening and thereby it is difficult to crystallize a substrate on which unit LCD panels of various sizes are formed. That is, if the length of the unit LCD panel is longer than the length of the opening of the mask, the crystallization has to be performed by partially overlapping the crystallized regions when stepping the substrate. Also, other LCD panels arranged on the substrate can be partially crystallized, thereby creating difficulty in performing a uniform crystallization.
Also, since the mask having the opening of a fixed size is applied, the mask has to be replaced into another mask when crystallizing a unit LCD panel of another size.